Is it possible to bake to a texture with animatable alpha gradient?
I had no success with baking, but could achieve the desired effect by rendering the image.


Set the materials alpha channel to initial transparency




Materials properties


Add a plane with an alpha gradient as texxture behing the scene


In Gimp create an image with the required dimensions
From the layer context menu select  Add Layer Mask

Use the Blend Tool to create tha gradient

Export to .PNG

Set camera to orthographic

Render and save the image.